Generally, a gallium nitride light emitting diode (GaN LED) is grown on a C-plane of a sapphire substrate. However, the lattice constant and the thermal expansion coefficient of the sapphire substrate are obviously distinguished from those of gallium nitride. Consequently, a strain between the gallium nitride epitaxy layer and the sapphire substrate is usually generated.
Moreover, because of the electric insulation property and the thermal insulation property of the sapphire substrate, the efficiency, life and brightness of the GaN LED are adversely affected. Consequently, the technology of growing GaN on sapphire is not satisfied.
Recently, researchers make efforts in the technology of growing gallium nitride on the gallium nitride substrate (i.e., GaN on GaN). Due to the lattice matching effect, it is presumed that the epitaxy layer grown on the GaN substrate has enhanced efficiency for the light emitting diode or the laser diode. However, it is difficult to fabricate the gallium nitride substrate, and the fabricating cost of the gallium nitride substrate is very high. In other words, mass production of the gallium nitride substrate is not feasible so far.
In addition to the sapphire substrate and the gallium nitride substrate, other technologies are researched. For example, the technology of growing gallium nitride on the silicon substrate (GaN on Si) and the technology of growing gallium nitride on the silicon carbide substrate (GaN on SiC) are related technologies.